1. Field of the Invention
The present invention relates to a rotor blade assembly for a gas turbine engine. In particular, aspects of the invention relate to a composite rotor blade assembly and arrangements for attaching composite rotor blade assemblies to a gas turbine engine.
2. Description of the Related Art
An axially rotating gas turbine engine comprises a number of rotor stages and a number of stator stages, each comprising aerofoil members mounted around an engine axis. The rotor stages are arranged such that, in use, the aerofoil members rotate around the engine axis. The stator stages are arranged such that, in use, the aerofoil members are fixed in position around the engine axis. Conventionally, a gas turbine engine would have a compressor and a turbine, each of which would comprise at least one (and usually several) rotor and stator stages.
A cut-away diagram of an example of a turbofan (bypass) gas turbine engine 10 is shown in FIG. 1. The gas turbine engine 10 comprises, in axial flow series, a fan 12, a compressor 60, a combustor 15, and a turbine 70, each of which operates in a conventional manner. The compressor 60 and turbine 70 are conventionally referred to as being part of the core of the engine, and the flow from the fan 12 that does not pass through the core of the engine is said to pass through a bypass duct 22.
The compressor 60 and turbine 70 both have rotor stages and stator stages. The fan 12 may be considered as a rotor stage, and may be referred to as a fan 12 or a fan stage 12. In the FIG. 1 example, a stator stage 32 is provided in the bypass duct 22 for the flow that does not pass through the core of the engine. This stator stage in the bypass duct 22 may be referred to as an outlet (or outer) guide vane (OGV) stage 32.
The fan 12 may be considered as a low pressure compressor. The fan 12 comprises a plurality of fan blades 120/190. In FIG. 1, the fan blades 120/190 may either be conventional fan blades 190 or fan blades 120 in accordance with aspects of the invention. Conventionally, the fan blades 190 are metallic items that are manufactured as single pieces and then assembled onto a rotatable shaft. In order to fix the conventional fan blades 190 onto the rotatable shaft, a wedge shape is generally provided at an inner radius of each blade, which is then slid into position and secured in place during assembly. Such a wedge shaped fitting arrangement may be referred to as a dovetail fixture.
However, metallic fan blades 190 and the conventional arrangements for assembling metallic fan blades 190 to form a fan 12 are very heavy.
It is desirable to replace the conventional metallic fan blades 190 with lighter components. In turn, this would result in a lighter fan stage, and thus a lighter engine, and improved fuel consumption (or SFC).
It is proposed to use a composite structure rather than a metallic structure to form the fan blades. Such a composite structure may have sufficient strength, but considerably lower weight than a conventional metallic fan blades. Composite fan blades may also be less expensive to manufacture than metallic fan blades.
However, conventional approaches to fixing the fan blades to the engine in order to form the fan may not be appropriate when using composite fan blades. For example, a dovetail style fixture may be difficult and/or expensive to form using a composite material whilst retaining the necessary strength and/or longevity of the fixture. Such a dovetail-type fixture may require a rapid build-up of thickness of the composite material in a particular area, making the manufacturing process more complex and potentially introducing weakness at ply drop interfaces.
Although much of the discussion above and elsewhere herein focuses on the fan blades and the fan stage, it will be appreciated that the same or similar considerations may apply to other rotor stages and blades therefor.